1. Field of the Invention
Nano-layered sorbents for CO2 capture, for the first time, were developed using layer-by-layer nanoassembly. A CO2-adsorbing polymer and a strong polyelectrolyte were alternately immobilized within a porous sorbent substrate. The solid sorbents of the present invention have fast CO2 adsorption and desorption properties and their CO2 capture capacity increased with increasing number of nano-layers of the CO2-adsorbing polymer.
2. Description of the Background Art
The emission of fossil fuel CO2 to the atmosphere is implicated as the predominant cause of global climate change; therefore, advanced CO2 capture technologies are of the utmost importance. Fossil fuels are the main energy supply in the world. However, the emission of CO2 from fossil fuel combustion has raised great concerns about the relation between anthropogenic CO2 and global warming.
According to the Energy Information Agency, approximately 40% of the U.S. CO2 emission is associated with electricity generation. Consequently, the capture and sequestration of CO2 from power-plant flue-gas streams is an essential scenario for carbon management. Current post-combustion CO2 capture and sequestration technologies require three main steps: (i) capture CO2 from the stack gas, (ii) compress the nearly pure CO2 to about 2,000 psi, and (iii) permanently “bury” or store the CO2 in certain geological structures deep in the earth. These processes can require up to one-third of the produced power-plant energy, which would otherwise be used as electrical energy for customers. Most of the energy cost of the three steps lies with step (i), i.e. CO2 capture. Monoethanolamine (MEA), used as a major aqueous wet scrubbing solvent, has high operating costs due to their heat of sorption plus the sensible and latent heating of the solution. The latent and sensible heating accounts for approximately ½ of the total regeneration energy for conventional liquid solvent systems. The presence of H2O, ˜70 wt. % in the MEA-based solvent, is a major cause of energy usage above that required for simple desorption of CO2. The energy penalty associated with solvent regeneration can be reduced by concentrating the amine solution, thereby reducing the sensible and latent energy needs connected with the water. However, highly concentrated MEA may lead to equipment-corrosion problems and unwanted foaming. Facing these facts and challenges researchers have recently proposed the concept of solid sorbents for CO2 capture. Compared to liquid amines dissolved in water, the solid sorbents may avoid much of the latent heat duty connected with aqueous solvent regeneration. Studies have indicated that solid sorbents may have the potential to require substantially less energy (e.g. a reduction of 30-50%) for regeneration than the current MEA-based CO2 scrubbing processes.
Carbon dioxide (CO2) is considered to be one of the major greenhouse gases directly influencing global climate change and human health, as more than 30 billion tons of anthropogenic CO2 is annually added to the atmosphere, and its emission is continuously increasing. The United States is the 10th largest emitter of CO2 emissions per capita as of 2004 (Raupach et al., 2007). Due to the continuous rise of CO2 in the atmosphere, extensive interest has been shown in developing carbon sequestration technologies, which may trap and store large quantities of CO2 from concentrated sources such as power plants. Capture is a key step in the overall carbon sequestration technologies. Various approaches have been reported for capturing and separating CO2 such as aqueous amine sorption, membrane separation, and cryogenic separation. Aqueous amine absorption has already been applied on the commercial scale for CO2 separation from power plant flue gas. However, aqueous amine absorption has high operating costs due to their heat of sorption plus the sensible and latent heating of the solution. Extensive water is required to prevent equipment corrosion and avoid airflow problems, which consequently results in high energy requirements for regeneration. Meanwhile, loss of amine components due to degradation and evaporation at moderate temperatures is another problem for aqueous amine absorption).
To avoid these problems, solid sorbents have been developed, e.g. amine immobilized materials including silica, fly ash, molecular sieves, activated carbons, and polymer supports. These amine solid sorbents offer several advantages including their capability to be used at low pressure for CO2 recovery, low capital cost, and low regeneration energy compared to aqueous amine solvents. Amines can be physically absorbed or chemically bonded in porous sorbents, and CO2 capture is mainly based on the interaction between primary and secondary amines with gaseous CO2 molecules. Although macromolecular amines may have better regeneration capacity and thermal stability, amines with low molecular weights have been primarily studied to date.
While solid sorbents may reduce regeneration energy, it is advantageous that the sorbent structure be tailored to minimize diffusion resistance, during both the CO2 capture, and subsequent regeneration. Minimal diffusion resistance will insure that the highest CO2 capacity is achieved in the shortest time—reducing the needed size of reactors and materials handling. Thus, it would be desirable to develop a technique where adsorbent chemistry could be deposited on high surface area supports, while creating uniform layers of desired thickness.